The Japanese market generates about 70% of revenue for Aflac. Japan Post, which also operates a bank and insurance unit, sells Aflac’s products in more than 20,000 post offices.

Last month, Japan Post’s insurance unit revealed it is examining 183,000 policies sold in the past five years amid concerns some of these sales could have involved improper practices such as charging premiums twice or prohibiting customers from switching policies.

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“There are limits to the information that we have in relation to Japan Post,” Mr. Crawford said, describing Aflac’s distribution partner as potentially distracted with resolving the issue.

Aflac said in a filing with regulators Thursday that it is reviewing more than 106,000 policies sold through Japan Post between May 2018 and May 2019 to determine whether customers were improperly charged. “If there is anything that needs correcting, we will do it,” Mr. Crawford said.

The insurer also said its sales through Japan Post could fall as much as 50% this year compared with last year. Sales are currently 25% lower than usual, Aflac said in its filing. The company sees the decline in sales as a temporary issue, Mr. Crawford said.

Aflac’s share price fell 5.6% Wednesday following news reports about the continuing investigation. On Thursday, the stock closed up 1.1% at $49.51.

Mr. Crawford said he sees it as his duty to pose tough questions in relation to the matter. “The biggest mistake you can make as a company is somehow to be soft or lazy on your practices,” he said.

Mr. Crawford is now facing a balancing act. “If he successfully navigates this challenge and keeps shareholders, regulators and policyholders aligned, this will be another feather in his cap,” said
Greg Peters,
an analyst at Raymond James Financial Inc.

Mr. Crawford became Aflac finance chief in June 2015, and before that served as CFO at CNO Financial Group since 2012.

The company generated $15.8 billion in revenue in Japan in 2018, while total revenue was $21.8 billion, according to Aflac’s annual report.

Mr. Crawford said the company’s management team has regular meetings with its counterparts at Japan Post but that the frequency of these interactions hasn’t increased as a consequence of the investigation at the Japanese distribution partner.

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